In the 3GPP LTE, a two-layer flat network architecture is adopted in the core network, i.e., the four network units of NodeB, RNC, SGSN and GGSN in the WCDMA/HSDPA stage are evolved into such two as the eNodeB, viz., the evolved Node B (eNB) (‘Base Station’ for short hereinafter), and the access gateway (aGW). And the fully IP distributed structure is adopted in the core network to support IMS, VoIP, SIP and MobileIP, etc.
FIG. 1 illustrates the network structure in an LTE system. The aGW may establish connections to multiple eNBs (e.g., eNB1, eNB2 and eNB3) through interface S1. And the eNBs may establish connections with each other in mesh (the dashed line in FIG. 1) through interface X2. The cells of eNB1˜3 have some illustrative user equipments as UE11˜E12, UE21˜23 and UE31˜33 respectively.
In LTE system, OFDM is adopted as the physical layer downlink transmission scheme for radio interface, and SC-FDMA is adopted as the uplink transmission scheme. With the application of OFDM, the same radio signal in different cells can be naturally combined in the air to improve the signal strength without any extra processing overhead, as is called the radio frequency combining (RF combining).
Therefore, the requirement to improve the gains on cell boundaries by supporting in-the-air RF combining under single-frequency network (SFN) multiple-cell transmission mode is defined as a baseline for the EMBMS in LTE, for it is necessary for EMBMS to transmit the same service data to different UEs.
The physical layer frame timing synchronization has been achieved for an eNB in the SFN with the precision satisfying the RF combining requirement for EMBMS. However, to guarantee the effectiveness of RF combining, the radio signals to be combined are required to be MBMS service content synchronous and consistent. That is to say, layer 2 (L2) transmission synchronization should be guaranteed for MBMS service's multi-cell transmission.
In addition, in LTE network architecture design, IP multicast transmission has been extended to eNB level in LTE architecture. The MBMS packet will be sent only once to a group of eNBs using IP multicast transmission. And current IP multicast routing protocol can guarantee that the route between each eNB and aGW mainly depends on network topology deployment and will not change unless the involved routers collapse. This instance will rarely happen. Besides, the router processing capability and transport network loading will be optimized during the network planning. So the only fact of the different transmission time delay is the different transmission route from aGW to eNBs. That is to say in spite of physical layer time synchronization in SFN area, different eNBs may receive the same MBMS packet at different time by different route.
FIG. 2 illustrates the transmission delays that the same data packet is transmitted from the aGW to different eNBs. As shown in FIG. 2(a), the routing through which the data packet is transmitted from aGW to eNB1 is: aGW==>router R1==>eNB1. And the routing through which the data packet is transmitted from aGW to eNB2 is: aGW==>router R2==>router R3==>eNB2. Different delays are resulted from that the same data packet is transmitted through different paths.
As shown in FIG. 2(b), at time T0, the data packet is transmitted from aGW to eNB1 and eNB2 respectively. It reaches eNB1 at time T1, and reaches eNB2 at time T2. Therefore, delay TD=T2−T1 causes to the transmission of the same data packet to different eNBs.
In this way, if the data packet is transmitted out just after it is received respectively by eNB1 and eNB2 from aGW, clearly it is asynchronously transmitted by different eNBs to UE. This results in that these data packets can not be combined correctly, or even causes extra interference. Moreover, after the same data packet arrives at the eNBs, it is necessary for each eNB to perform such operations as segmentation, coding and modulation and so on for frame construction. Inconsistent framing time will also affect these data packets' RF combining.